Pulse width modulation has been used as a motor speed control technique for a number of years. Typically, such an approach requires a transformer and a power supply to convert the A.C. line voltage into a D.C. voltage which is selectively applied to transistors so as to control which transistors conduct and the length of time that they conduct. The conduction of one pair of transistors causes the motor to rotate in one direction whereas the conduction of another pair of transistors causes motor rotation in the opposite direction. In both cases, the length of time that the transistors are conducting determines the speed of the motor. While this technique provides good speed regulation and control, it requires a transformer and power supply which can be quite bulky.
A variation of the foregoing utilizes SCR's in a back-to-back relationship. The motor rotates in one direction when the first SCR is triggered during the positive half cycle of the A.C. line voltage, and rotates in the opposite direction if the second SCR is triggered during the negative half cycle of the A.C. line voltage. In either case the speed of the motor depends on the "firing angle" of the SCR. This method eliminates the need for a transformer and power supply but is susceptible to drift and instabilities particularly around the zero speed point, which is of primary concern in a servo positioning system.
Because of the foregoing, it has become desirable to develop a motor speed control system which can be powered directly from the incoming power line without the need for a transformer and power supply, which is not susceptible to drift and instabilities, and which can be adapted for use with series wound, or shunt wound A.C. motors or D.C. motors.